<template>
    <ul class="list">
        <li class="item"
            v-for="item of letters"
            :key="item"
            :ref="item"
            @touchstart="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchend="handleTouchEnd"
            @click="handleLetterClick"
        >
            {{item}}
        </li>
    </ul>


</template>
<script>

export default {
  name: 'CityAlphabet',
    props: {
      cities:Object
    },
    computed : {
        letters () {
            const letters = []
            for(let i in this.cities){
                letters.push(i)
            }
            return letters
        }
    },
    data () {
      return {
          touchStatus:false,
          startY:0,
          timer:null
      }
    },
    updated () {
      //updated :因为cities一开始是空对象，当axios请求回数据然后修改了 cities的值之后，该生命周期函数就会执行
      //得到 A 到 导航栏的距离（即搜索框下面一点点的蓝框边缘）
        this.startY=this.$refs['A'][0].offsetTop
    },
    methods : {
        handleLetterClick (e) {
            const letter=e.target.innerText
            this.$emit('change' , letter)
        },
        handleTouchStart () {
            this.touchStatus = true
        },
        handleTouchMove (e) {
            if(this.touchStatus){
                if(this.timer){
                    //当手指触碰到另外一个字母时，就停止上一滚动操作，直接执行当前手指触碰操作
                    clearTimeout(this.timer)
                }
                this.timer=setTimeout(()=>{
                    //手指当前位置到导航栏底部边缘的距离
                    const touchY=e.touches[0].clientY-79
                    const index=Math.floor((touchY-this.startY)/20)
                    if(index >=0 && index <this.letters.length){
                        this.$emit('change',this.letters[index])
                    }
                },16)
            }
        },
        handleTouchEnd () {
            this.touchStatus = false
        }
    }
}
</script>

<style lang="stylus" scoped>
@import "~styles/varibles.styl"
    .list
        display:flex
        flex-direction:column
        justify-content:center
        position:absolute
        top:1.58rem
        right:0
        bottom:0
        width:.4rem
        .item
            line-height:.4rem
            text-align:center
            color:$bgColor
</style>
